Coated abrasives or abrasive products, sandpaper being a common example, consist of a substrate backing, abrasive grains and a bonding system which operates to hold the abrasive grains to the backing. For a typical coated abrasive product, the backing is coated with a first layer of adhesive, commonly referred to as a "make coat", and then the abrasive grains are applied. The adherence of the resulting adhesive/abrasive combination or composite is then generally solidified (i.e., set) enough to retain the abrasive grains to the backing, so that a second layer of adhesive, commonly referred to as a "size coat", can be applied. The size coat further reinforces the coated abrasive product. Once the size coat is solidified (set), the resulting coated abrasive product can be converted into a variety of convenient forms for various uses, for example sheets, rolls, belts, and discs. Generally, the size coat and make coat may be the same, although they do not necessarily comprise the same adhesive or very similar adhesive compositions. Solvent dilutions to achieve convenient viscosities may differ for them.
The substrate, for typical coated abrasive products, is typically paper, a polymeric film, cloth, a fibre web such as a vulcanized cotton fibre web, a nonwoven web, combinations or composites thereof or treated versions of these. Commonly used abrasive grains include: flint, garnet, emery, silicon carbide, aluminum oxide, ceramic aluminum oxide, alumina zirconia or multi-grain granules. Conventional bond systems typically comprise a glutinous or resinous adhesive, and optionally include a filler. Examples of common adhesives are: hide glue, phenolic, urea-formaldehyde, melamine-formaldehyde, epoxy, varnishes, acrylate resins or combinations thereof.
Fillers are typically inorganic particulate material which has been dispersed within the resin. Fillers operate to inexpensively increase the volume of resin, thus decreasing costs. Also, fillers often make the cured resin harder, more heat resistant and/or less likely to shrink when set. The latter is important, since shrinkage during setting causes considerable stresses in the product. In some instances fillers may also be used as pigments. Fillers are typically of small particle size, are relatively soft by comparison to abrasives, and do not themselves cause much abrasion in use.
Generally fillers comprise materials which are substantially inert, or non-reactive, with respect to the grinding surface; the grinding surface being the surface acted upon by the abrasive product in use. Occasionally, however, active (i.e. reactive) fillers are used. These fillers interact with the grinding surface during use, in beneficial manners.
U.S. Pat. No. 2,322,156 discloses the use of fillers in glutinous and resinous adhesives to improve their hardness, heat resistance, and moisture resistance and to lower their overall cost. The patent refers to typical fillers as being inert, relatively nonabsorbent, nonfibrous, hard, dense, inelastic and nondeformable materials.
U.S. Pat. No. 2,534,805 discloses the use of a laminating adhesive filled with an inert, relatively nonabsorbent, nonfibrous filler. The modified adhesive, according to the patent, is used to laminate two backings together. The addition of filler to the adhesive apparently substantially lowered the rate at which the modified adhesive expanded or contracted, due to changes in humidity.
U.S. Pat. No. 2,873,181 teaches the use of wollastonite, i.e. calcium silicate, as a filler for glue or synthetic resins used in coated abrasives.
The abrasive coating (i.e. abrasive/adhesive composite attached to the substrate) for abrasive products is typically relatively thin, often essentially a monolayer of abrasive particles. The thickness for typical commercial products is often on the order of 0.01-2.0 mm. Thus, even a relatively small, localized failure in the bonding system can easily lead to an exposure of a portion of the substrate, and thus a substantially complete failure of the product, in use. It is noted that coated abrasive products are typically used under conditions of relatively high pressure and temperature; for example at a point of engagement between a coated abrasive belt and a grinding surface. Pressure-generated and/or heat-generated stresses can facilitate failure of the bonding resin to retain the abrasive on the substrate, and thus failure of the product.
Coated abrasives such as sandpaper differ significantly from grinding wheels. For example, grinding wheels are typically formed as a relatively deep or thick (three-dimensional) structure of abrasive grains or particles adhesively retained together in a wheel. A minor failure in adhesive poses relatively little problem, since only an outermost layer of abrasive grains would be affected. That is, a lower, and still effective, layer of abrasive would be exposed. Also, coated abrasive products generally involve a relatively high volume ratio of adhesive to abrasive, by comparison to grinding wheel, and hence greater opportunities for stress to be imparted to the adhesive.
Many coated abrasive products are used or stored in high humidity environments, or are used under a water flood or wash, or are themselves washed between uses. Almost all commonly used resinous adhesives are sensitive to water. Under relatively wet conditions, typically used conventional bond systems substantially weaken. Thus, the coated abrasive product, in some cases, may fail because the bond system has been sufficiently weakened by water that it can no longer hold the abrasive grains or particles to the backing.
Past attempts at improving the performance of bond systems in coated abrasive products have generally focused on improving the bonding interaction between the abrasive and the adhesive. That is, it has generally been believed that failure to obtain good, water resistant chemical adherence between the resin and the mineral has been the problem. The present invention concerns a unique approach to improving coated abrasive products and/or their manufacture, whereby the bonding system is improved by improvement at the resin-filler interface, through use of coupling agent(s).